Young brothers Nick and Theo Kennedy love watching football on Sundays, but the afternoons are anything but relaxing. When the Patriots are on, the TV in their Westwood living room is tuned to that game, but they regularly flip to the NFL’s RedZone, a live highlights channel, and each boy constantly checks his tablet, monitoring ESPN for updates on more than 50 players leaguewide.

No money is at stake. But like a growing number of children, Nick, 13, and Theo, 11, are playing for something more important: fantasy football bragging rights. “We like to razz each other at school,” Theo said.

About 25 million people, ages 12 and over, play in fantasy football leagues around the United States, according to Paul Charchian, president of the Fantasy Sports Trade Association
. That’s up from about 16 million in 2007. And kids make up the second-biggest age category, behind 18- to 35-year-olds. Charchian estimates that almost 3.5 million kids play fantasy football and says their numbers are growing as the sport charms a football-obsessed nation.

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Back in 2003, the number of kids playing fantasy football was “nonsignificant,” according to Kim Beason, a University of Mississippi professor who has conducted surveys on the sport. In his most recent survey, in 2010, one quarter of the almost 1,100 respondents reported that their sons were playing, as were 4 percent of their daughters. More kids are playing, he said, because fantasy football has shed its nerdy image. “It was geek originally,” Beason explained, “because of the statistic-heavy way the game was played [pre-Internet]. That has disappeared using today’s technology.”

Fantasy football has gone so mainstream, Charchian said, that playing has become a way of keeping up with friends. “Dropping a league is like ending a dozen friendships.”

For those who aren’t on a fantasy football team, here’s how it works: Players join or create a league, and then get to act like owners of actual teams, “drafting” real NFL players and making game-day decisions. Points are earned on the basis of how the players perform in their real-life games that day.

The demand from the youth market is so strong that in 2007 the NFL added a kid-focused fantasy game on its popular NFLRush site, and this year launched mobile apps, the better to check on how your players are doing from the sidelines of travel soccer, or, like Christian Abbate, a Hanover 15-year-old, while you’re out to dinner with your grandfather.

But with fantasy football a constant source of youth conversation, a kid needs to do what a kid needs to do. “It’s a superstitious thing for me,” Abbate explained. “I think by checking, I’ll help my players.”

Here’s another sign that fantasy football has taken hold of the kiddie set: It has become a source of friction between kids and parents. On a recent morning in Newton, Bret Miller, at 11, already a four-year fantasy veteran, and his father, Andrew, were bickering over a player the elder Miller had insisted they acquire for the team the two comanage.

“I look at my dad’s sites and they help me,” Bret said when asked where he gets his football information.

Yahoo! Sports fantasy expert Brad Evans calls fantasy football the “sports cards of the 21st century.” Considering that cards were just cards, and that fantasy football has invaded so many aspects of modern life, from smartphones to sitcoms, it’s like cards on steroids.

Although parents rightfully worry about all sorts of online distractions, many take solace in fantasy’s potential to teach math and strategic skills.

In the mid-1990s, Dan Flockhart, a middle-school math teacher in Northern California, increased his students’ motivation by combining math and fantasy football.

“They consistently tested in the 80th percentile and above against other private-school students,” he said.

“When I wrote the first book, people thought I was nuts,” he said, pointing to two University of Mississippi surveys, in 2007 and 2009, that found fantasy helped with math skills and made learning more enjoyable.

And there’s another plus, he added.

“I have had fathers tell me that fantasy sports has given them quality time with their adolescent sons and daughters, who previously wanted nothing to do with them.”

The National Football League also plays up the educational component of fantasy football. The league awards a $10,000 college scholarship to the overall winner of the fantasy competition on the NFLRush site, and gives $1,000 scholarships to weekly winners.

Last year, with Tom Brady as his quarterback, Joseph Chiarenza, then 11, of Medford, won one of those $1,000 scholarships. Asked what advice he’d give other youth players, Chiarenza sounded like an athlete in a post-game press conferences. “Have fun,” he said.

Meanwhile, fantasy football has become so popular that Jeremy Rosenstock Doughty, a Brookline seventh-grader who plays in three leagues, says he’s surprised when he talks to a kid who doesn’t have a team.

“It’s such a part of me,” he said. “My dad plays in a couple of leagues, and I’ve always wanted to be just like him.”

At the same time, he added, he’d like to try to watch football for the beauty of the sport, rather than to see how his players are scoring.

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